The present invention relates to a printed wiring board, and more particularly to a printed wiring board to be connected with bump electrodes of a bare chip such as LSIs.
In prior art, a flip chip bonding has been used to mount a bare chip such as LSIs on a printed circuit board. In the flip chip bonding, a bare chip is bonded in face-down via bump electrodes thereof onto the printed circuit board so that the bump electrodes and various devices integrated on the bear chip face to the printed circuit board.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a structure of the conventional flip chip bonding. A bare chip 58 is mounted in face-down on a printed wiring board 51. The bare chip 58 has a face on which a plurality of bump electrodes 59 are provided. The printed wiring board 51 has a face on which bare chip mounting pads 52 are provided. An insulation film 54 extends over the surface of the printed wiring board 51 and the bare chip mounting pads 52 partially. Parts of the bare chip mounting pads 52 are not covered by the insulation film 54. The bare chip 58 is mounted in face-down on the printed wiring board 51 so that the bump electrodes 59 of the bare chip 58 are made into contact directly with the bare chip mounting pads 52 of the printed wiring board 51, whilst a space is formed between the bare chip 58 and the printed wiring board 51. The space is filled with a sealing resin 60.
As well illustrated in FIG. 1, the bare chip mounting pads 52 are covered by the bare chip 58, for which reason it is difficult to modify the wirings (not shown )extending to the bare chip mounting pads 52 in accordance with the requirement for modification to the logic design appeared after electronic devices have been mounted.
In the Japanese laid-open patent publication No. 62-60293, in order to overcome the above problems, it was proposed to a printed wiring board 61 as illustrated in FIGS. 2A and 2B. FIG. 2A is a plane view illustrative of the printed wiring board. FIG. 2B is a plane view illustrative of the printed wiring board on which an electronic device is mounted. In FIG. 2A, a plurality of mounting pads are aligned over the printed wiring board 61. Each of the mounting pads comprises a contact pad 62 and a wiring pad 63, both of which are unitary formed and the contact pad 62 is positioned inside and the wiring pad 63 is positioned outside. As illustrated in FIG. 2B, the electronic device 64 is mounted on the contact pads 62. Therefore, the wiring pads 63 are not covered by the electronic device 64. It is easy to connect the wirings to the wiring pads 63. The contact pads 62 and the wiring pads 63 are connected a pinched portion so as to suppress an excessive heat conduction from the wiring pads 63 to the contact pads 62 when a soldering process is performed. The contact pads 62 and the wiring pads 63 are made of a copper foil.
The above conventional printed wiring board has the following disadvantages. Since as described above the contact pad 62 as the mounting pad and the wiring pads 63 are unitary formed via the narrow portion, the total area of each of the above pads is larger than the normal mounting pad of the copper foil.
If the solders are previously applied on the mounting pads or the wiring pads 63 for subsequent bonding the electronic device 64, a large amount of the solder must be applied on the mounting pads having a larger area in order to ensure the necessary height of the solder. This results in an increased probability of bridging the solders in supplying the same. This may result in deterioration in quality of the printed wiring board.
Another problem is that after the electric device 64 has been mounted on the printed wiring board 61, the sealing resin is injected into the space between the bare chip and the printed wiring board. There is a possibility the sealing resin might flow onto the wiring pads 63 and cover the same. This prevents the provision of the wiring onto the wiring pads 63.
As the numbers of the connection pins and the connection terminals are increased, the pitch of the connection pins and the connection terminals becomes small. Therefore, the width of the contact pads 62 must be narrow. The narrow width of the contact pads 62 results in an increased probability of inadvertently disconnecting the pinched portion between the wiring pads 63 and the contact pads 62. Thereby decreasing the reliability and yield of the printed wiring board.
In the above circumstances, it had been required to develop a novel printed wiring board free from the above problems.